Some conventional high performance variable gain amplifiers (VGAs) use dominant fixed resistors at VGA input and output to try to minimize input and output impedance variations. For example, if a change in gain would otherwise result in a change in impedance from 10 to 100 Ohms, adding a dominant 1000 Ohm resistor in series would reduce the overall variation to less than 10%. However, this impedance control technique dramatically degrades noise and/or linearity performance of VGAs.
Other conventional high performance VGAs utilize a variable attenuator followed by a fixed gain amplifier. As a result, as gain decreases the noise figure (NF), which measures degradation of the signal-to-noise ratio, of conventional VGAs degrades decibel per decibel (dB per dB). In other words, the NF increases approximately one dB for every dB decrease in gain of a conventional high performance VGA.
FIG. 1 illustrates a prior art variable gain current feedback amplifier disclosed in U.S. Pat. No. 6,906,595. As illustrated, prior art variable current gain current feedback amplifier 10 comprises operational amplifier A16, transistor Q15, input and output terminals, IN, OUT, fixed input and output resistors R11, R12, variable resistors R13, R14, and power, ground terminations VCC, GND. In particular, amplifier 10 is a variable current gain current feedback amplifier. The input impedance of amplifier 10 is primarily set by input resistor R11. This design results in relatively high noise due to the noise contribution of input resistor R1. An additional problem is that the output impedance varies as the current gain varies. This architecture requires additional circuits or stages shown in other drawings of U.S. Pat. No. 6,906,595 for output impedance matching.
A second prior art reference entitled, “A Wideband Low-Noise Variable-Gain BiCMOS Transimpedance Amplifier,” was presented by Robert G. Meyer and William D. Mack in IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 29, NO. 6. June 1994. This reference discloses a variable gain transimpedance amplifier. The architecture described by this requires an additional stage, i.e., an output stage, with a series resistor to provide output impedance matching.
The state of the art before the present invention is exemplified by the data sheet for Analog Devices part number AD8376, which, according to Analog Devices, is an “Ultra low distortion IF Dual VGA.” According to the data sheet for AD8376 (not shown), gain is varied by adding power loss at the input of a fixed gain amplifier. As a result, the noise figure degrades by 1 dB for every 1 dB step in gain. If the power loss is placed at the output of the fixed gain amplifier, it is the output IP3 (output third order intercept point or IP3o) that varies 1 dBm/dB instead of the noise figure. If the power loss is added at both input and output to vary gain, split the difference and lose 0.5 dB of NF AND 0.5 dB of output IP3 for each 1 dB gain step. Other state of the art VGAs are exemplified by data sheets for Maxim part numbers MAX2027 and MAX2055 (not shown).
Prior art VGAs are problematic because they fail to achieve high levels of performance with regard to each of impedance variation, NF and IP3. For example, some prior art designs sacrifice NF in favor of minimizing impedance variations. Some prior art designs implement lossy resistive networks at VGA input while others implement them at VGA output. A lossy resistive network at the input degrades NF while a lossy resistive network at the output degrades IP3. Thus, there is a need for a VGA that achieves high levels of performance with regard to each of impedance variation, NF and IP3.